Hear Again, if I were you I would ask your psychiatrist to explain it to you, or to point it out to you when you are doing it (if you do it).
The only other way I can explain is that it is a mental coping mechanism. You can feel like your body is not real, or like you are not inside your body, or your body is someone else's, or you are observing someone else's life. Physical sensations can be diminished or completely blocked out. Those are just some examples. Daydreaming is a life form a dissociation. Many people with chronic PTSD also use some form of dissociation, oftentimes stronger than daydreaming. The most extreme forms of dissociation manifest themselves as Dissociative Identity Disorder, in which a person can create two or more alters, each of them containing different memories and having their own personality patterns. With people who have two alters, often one of them contains flashback material and the other doesn't.